Docs: env var fallback for API keys

[jamesbraza]

I am pretty sure these are equivalent:

      - uses: promptfoo/promptfoo-action@v1
        with:
          openai-api-key: ${{ secrets.OPENAI_API_KEY }}
          anthropic-api-key: ${{ secrets.ANTHROPIC_API_KEY }}
          ...
        env:
          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
          OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}

Can we document that it's possible to use env vars, instead of openai-api-key/anthropic-api-key args themselves for API keys?